1. Field of the Invention
The present invention relates to compositions and methods of using the same for treating glass surfaces, such as glass containers, to improve their scratch resistance and lubricity while preferably preserving or improving other desirable properties in the resulting coating.
2. Description of the Prior Art
In the prior art, coatings have been applied to vitreous surfaces for various purposes. Such purposes include promoting adhesion between a vitreous surface and another substance, coloring the vitreous surface, controlling reflectance thereof, increasing electrical conductivity over the vitreous surface, increasing strength and durability of articles having such surfaces and increasing abrasion and scratch resistance of the surface.
The aforenoted vitreous surfaces are provided in a variety of configurations such as fibers, sheets, bottles and the like and the specific properties of the coating will in part be dictated and controlled by the particular configuration of the vitreous surface.
It is well known in the art that glass derives its strength from an unblemished surface and any scratches or flaws which are present on the surface decrease the strength of the glass many fold. Glass articles such as jars, bottles and tumblers possess their maximum strength shortly after they are formed. Their strength rapidly diminishes when they are subjected to abrasive contact in the course of handling, filling, packaging and shipping.
This problem is particularly acute when glass containers are used by processors of foods and beverages, because the glass containers are subjected to a number of processing operations wherein they are successively washed, filled, closed and packaged for delivery. In spite of precautions taken to minimize scratching and abusive handling, the many washing, sterilizing and other operations to which such containers are subjected cause the glass to be rubbed or otherwise contacted in various ways, often under high pressure, which can cause scratching. The glass containers repeatedly come into contact with each other as they move from station to station and as they are handled in the various operations. Breakage of the glass containers can cause a substantial increase in production costs, particularly if the breakage occurs after the containers have been filled.
Attempts to minimize the formation of defects in newly formed glass articles have included modification of the handling equipment and the application of various coatings to the newly-formed articles. While some coatings have resulted in a reduction in the number of defects, the coatings have created other problems such as discoloring of the glass or other changes in the appearance thereof, difficulty of application of the coatings, or lack of durability through washing and steam cleaning operations. In addition, many coatings were not acceptable for glassware because they were not approved for use with food or because labels did not tightly adhere to the coated surfaces.
Some of the compositions previously used as a protective coating for glass include tin and titanium compounds such as titanium and stannic halides and alkyl titanates which are sprayed on the surface of the glass at a temperature of from 900.degree. F. to 1200.degree. F. whereby said compounds react to form TiO.sub.2 or SnO.sub.2. The oxide layer is tightly adhered to the surface of the glass and thereafter the glass is cooled to about 450.degree. F. or lower and a second coating is applied consisting of polyethylene, as described in U.S. Pat. Nos. 3,323,889; 3,368,915 and 3,667,926. These coatings involve two separate applications, one at the hot end of the annealing lehr and another at the cold end of the lehr.
Bottle manufacturers have expressed a need to change coating systems employing tin oxide due to the expense of applying coatings in two separate applications and the problems associated with stannic halides. More specifically, stannic halides hydrolyze in the presence of water to form hydrochloric acid which corrodes metal equipment. This makes it necessary for bottle manufactureres to replace various line and plant components on a routine basis.
One attempt to circumvent the above problem is illustrated in U.S. Pat. No. 3,873,352 which is directed to glass articles coated with an aqueous composition containing a silane, such as polyethyleneiminepropyltrimethoxysilane or an ethylenediaminopropyltrialkoxysilane, and an aqueous emulsion of an olefin such as polyethylene. While good abrasion resistance and labelability are alleged for this coating the adhesion of the same to the bottle when subjected to an aqueous wash, and particularly an aqueous caustic wash, is too low for certain applications.
Thus, while coating compositions may impart good initial scratch resistant properties to glass containers, these properties would be considerably reduced, if not lost altogether, should the coating be delaminated in whole or in part when the treated glass surface is wetted by water or steam, during a processing cycle. For example, glass containers for beer are subjected to hot caustic washes prior to being filled and the danger of delamination is increased substantially.
Thus, the search for a suitable protective glass bottle coating has been further complicated by the combination of properties which the same should possess in order for it to be a commercially acceptable alternative. For example, not only should a suitable protective coating exhibit good scratch resistance, lubricity, adhesion to the bottle, and water resistance, but it should also be capable of providing a surface to which common label adhesives can be applied and adhered to. The coated surface should also be clear and remain clear after processing.
Silane containing epoxy compositions used as protective coatings for materials, such as for example glass fiber, are disclosed in U.S. Pat. Nos. 3,169,884; 3,211,784; 3,297,186; 4,049,861; 4,069,386; and 4,081,421. However, none of the above patents employ in the coating formulations described therein waxes, surfactants or the specific type of amine terminated epoxy formualtions described herein.
Water dispersible epoxy compositions are disclosed in U.S. Pat. Nos. 3,336,253; and 3,449,281 as well as commonly assigned U.S. Patent Application Ser. No. 069,969, filed Aug. 27, 1979 entitled "Two Component Aqueous Based Can Coating Composition" by D. Shimp, D. Hicks, and R. Graver the disclosure of which application is herein incorporated by reference. None of these two patents and applications show the use of said formulations to coat glass bottles and the like and the compositions described therein do not possess the combination of properties obtainable from the specific formulations described herein when used as a coating on glass bottles.
U.S. Pat. No. 4,056,208 is directed to caustic-resistant coatings of glass which comprise a primer layer of an epoxy resin and an organo functional silane (which can be applied as a dispersion), and a top coat of an ionic copolymer of alpha-olefins and alpha, beta-ethylenically unsaturated carboxylic acids. The presence of the top coat negates the need to develop properties such as label adhesion, scratch resistance and lubricity in the primer coat and the latter is used only to provide a means for increasing the adhesion of the top coat to the glass via the primer coat.
The search has therefore continued for bottle coating formulations which possess the aforedescribed desired properties. The present invention is a result of this search.
It is therefore an object of the present invention to provide an aqueous based coating composition which exhibits improved and durable adhesion to glass surfaces and renders the same highly resistant to scratches, while simultaneously maintaining or improving the strength characteristics of the glass.
It is another object of the present invention to provide an aqueous based coating composition which yields a coating having improved scratch resistance, lubricity, quick set time, blush resistance, and label adhesion when applied to glass surfaces.
It is another object of the present invention to provide a method for forming scratch resistant coatings on glass surfaces while avoiding the shortcomings of the methods and compositions known and used in the past in attempting to form such scratch resistant coatings.
It is still another object of the present invention to provide a method for coating glass containers to improve their scratch resistance, and lubricity and yet provide the adhesion of conventional label adhesives thereto.
Still another object of the present invention is to provide a method for coating a glass surface, such as the exterior of a glass container, so as to impart thereto at least improved dry and wet scratch resisting properties, thus permitting the container to undergo normal handling, processing, and shipping, with the consequent rubbing of the glass surface with other surfaces, without materially decreasing the strength of the container.
A further object of the present invention is to provide an article of manufacture having a glass surface and, on said surface having a tightly adhering, thin, preferably transparent coating which coating imparts at least superior wet and dry scratch resisting properties to the glass surface, said coating being highly blush resistant.
Still another object of the present invention is to provide glassware having a tightly adhering, thin, substantially colorless and transparent coating on its surface, which coating imparts increased strength and resistance to the ware, enabling it to withstand greater internal pressures without breakage.
A further object of the present invention is to provide glassware having a thin, substantially colorless and preferably transparent coating on its surface which coating is insoluble in water, is free from taste and odor, and is nontoxic, so as to permit the ware to be safely used for food and beverages.
These and other objects and features of the invention will become apparent from the claims and from the following description.